Gallium-nitride based high electron mobility transistors (HEMTs) have attracted a lot of interest for high-frequency and lately also high-power applications because of their potentials for fast and low-loss switching, high breakdown voltage and high operating temperature.
However, AlGaN/GaN HEMTs with Schottky gates and without passivation suffer from high gate leakage, current dispersion and a variety of reliability issues.
EP 1612866 discloses that silicon nitride, especially when deposited in situ with the III-nitride layers in a metal-organic chemical-vapor deposition (MOCVD) reactor at high temperature forms a high-quality passivation layer that reduces the relaxation, cracking and surface roughness of the AlGaN. It also neutralizes the charges at the top AlGaN interface and forms a high-quality interface with low interface state density.
For power applications it is important to reduce the gate leakage current to minimize the power consumption in the off-state. To achieve fast turn-on and turn-off switching a large gate bias swing is needed. For this reason low gate leakage is essential both in reverse and forward gate biasing, the latter being in particular important for enhancement mode (e-mode) devices. To suppress the gate leakage current a gate dielectric is often inserted between the Schottky gate and the AlGaN barrier and a metal-insulator-semiconductor (MIS) transistor is fabricated.
However, an ideal gate dielectric has a high dielectric constant because devices with a higher transconductance can be achieved. Moreover, to suppress the leakage, a large band-offset energy is required at the insulator/AlGaN interface. From this viewpoint the dielectric constant for Si3N4 (∈˜7) is not high enough compared to that of AlGaN compounds (∈˜9). Also the bandgap of Si3N4 (˜5 eV) is not much higher compared to AlGaN (˜4 eV).
Al2O3 is one of the most attractive dielectrics applicable to power MIS devices because of its large bandgap (˜7 eV), relatively high dielectric constant (∈˜9) and high breakdown field (˜10 MV/cm). Best quality Al2O3 films are deposited by atomic layer deposition (ALD) but the density of interface states (DIT) at the AlGaN interface is typically very high: 1×1012 cm−2 eV−1 or higher.
Maeda et al (Appl Phys Lett 87, 073504 (2005)) discloses a MIS-HFET having a Al2O3/Si3N4 gate insulator, with Si3N4 in contact with AlGaN and deposited only on the gate region, i.e. under the gate metal. Between source (drain) and gate regions Si3N4 and SiO2 layers were successively deposited as surface passivation. All the insulators were deposited by electron cyclotron resonance (ECR) sputtering.
Nowadays GaN transistors are typically fabricated on small area 2 inch or 3 inch diameter SiC or sapphire substrates. The Ohmic source-drain and Schottky gate electrodes are usually formed by Au-containing metallization stacks that are patterned by contact lithography followed by metal lift-off.
However, to compete with Si technology, the reduction of the cost is a key factor. For this reason, GaN epitaxial grown material on large diameter 150 mm, 200 mm or even 300 mm Si substrates is being developed. The new transistor fabrication technology should be Si-CMOS compatible using stepper lithography, Au-free metallization schemes and metal patterning by dry etching.
None of the methods referred above is suitable for use in a Si-CMOS compatible scheme.
Therefore it is desirable to have a manufacturing method for a HEMT device which is compatible with Si-CMOS process technology, enabling the use of e.g. stepper lithography, Au-free metallization schemes, and/or metal patterning by dry etching.